I used ABS pipe to make a frame and covered it with plastic sheeting. Make it triangular to allow the snow to slide off.

If you only want to keep the area around the deicer melted, make a cover from an appropriately sized Roughneck container attached to a slab of Styrofoam. The top edge of the container should be inserted through a hole in the Styrofoam. You can use Greatstuff foam to seal the two together. Float the container upside down,sort of like the Monitor(sometimes referred to as a cheesebox on a raft) from the Civil war. The foam collar should be at least 4 to 6 inches wide all around. This stabilizes the container. Even if the contraption is sealed in with snow and ice the interior will be mostly ice free. Any snow on top will melt.

This is the basic form of the homemade lightbulb deicer which is cheaper to run than most deicers.